The present invention relates in general to control systems and in particular to a decentralized, cautious adaptive control system.
A "plant" can be characterized by an "open-loop" transfer function relating its outputs to its inputs. A "feedback controller" monitors one or more outputs of the plant and adjusts one or more control inputs so that the outputs are driven to desired states. A feedback controller also has a transfer function relating its sensor inputs to its control outputs. The "closed-loop" transfer function of a plant augmented by a feedback controller is thus a function of the transfer functions of both the plant and the controller. When the open-loop transfer function of a plant is known and constant, the transfer function of a feedback controller may be selected so that the closed-loop transfer function of the plant optimizes plant response with respect to various criteria, including system stability. However, the open-loop transfer function of many plants can change, and the feedback signal provided to the controller may make the plant unstable.
An "adaptive" feedback controller has a transfer function that changes to compensate for changes in the open-loop transfer function of a plant in order that plant operation remains stable. An adaptive controller must be able to identify the open-loop transfer function of the plant and adjust its own transfer function so that the closed-loop transfer function of the plant satisfies predetermined criteria. However, the identification of the plant's new open-loop transfer function following an abrupt change can be time consuming, and, until the new open-loop transfer function is accurately identified and the transfer function of the controller is appropriately adjusted, the output signals produced by an adaptive controller may still cause the plant to become unstable.
The paper "Adaptive Flutter Suppression in the Presence of Turbulence", by Abhijit Chakravarty, Dagfinn Gangsaas, and John B. Moore, published in June, 1986 by the American Control Conference, and incorporated herein by reference, describes a "cautious" adaptive flutter controller that monitors output signals of accelerometers mounted on the wings of an aircraft and produces output feedback control signals for the aircraft's ailerons for dampening detected wing flutter. Following an abrupt change in an aircrart s response to aileron controls due, for example, to sudden wing damage the flutter controller temporarily attenuates its output feedback control signals so that the feedback signals do not strongly affect aileron movement, thereby avoiding the possibility that the feedback signals will cause the aircraft to become unstable. As the controller acquires more "confidence" in its identification of the plant's new open-loop transfer function, it decreases the amount of feedback signal attenuation whereby the feedback signals more strongly influence aileron movement. The cautious adaptive control system described in the referenced paper thus prevents the controller from destabilizing the aircraft following a change in its open-loop transfer function